Doesn't twenty ships seem like a bit of a measly force to go and attack the Founders? I mean, okay it was a sneak attack that wasn't suppsed to fight its way to the home world or anything... but look at the fleets we see during the war, hundreds of ships strong or more. Even a pretty low level orbital defence system would be more than capable of handling only twenty ships, and the Founder's planet would certainly be likely to have one of those.

Factoid :

The single shot of the Defiant ramming through a Jem'Hadar ship took more than four days to shoot.

Plotline

With twenty ships on their way to the Omarian Nebula, Garak is ordered to torture Odo for information about the Founders. Back in the Alpha Quadrant Sisko is ordered to remain at DS9 with the Defiant; but as the fleet begins its bombardment the Dominion springs a deadly trap.